Iron Substrate Research Articles

Use of additives to improve bonding strength of the adhesive prepared from used polymer: Sustainable management approach

In the present study, value added and environment-friendly adhesive material is prepared from post-consumer (used) expanded polystyrene (PCEPS) for possible applications. The physico-chemical properties of the prepared adhesives, such as, solubility, viscosity, shear strength/bonding strength, moisture content are estimated as per the standard procedures. The study shows that PCEPS is sufficiently soluble in various solvents, such as, m-xylene, n-BA, THF, MEK etc. Various substrate materials, such as, paper, wood, cement and clay bricks, and ceramic tiles are considered to check the suitability of the prepared adhesive to bind them. The study indicated that the prepared adhesive (PCEPS-MEK) is suitable to bind various substrate materials mentioned above. The study further extended to bind the metal (iron) substrates with PCEPS adhesive. From the results, it is noticed that PCEPS adhesive offered lower bonding strength over commercial adhesive Araldite. Therefore, the study furthermore extended to improve the PCEPS adhesive bonding strength using two different additives. The result implied that the bonding strength of the EPS adhesive decreased drastically with the increase of Additive-1 (A1). However, the bonding strength of EPS adhesive increased in presence of Additive-2 (A2) and its loading. Around 69% increment in bonding strength of PCEPS adhesive is obtained when the Additive-2 loading is increased up to 50 wt% under present experimental conditions. Therefore, Additive-2 can be used as one of the suitable additives to improve the PCEPS adhesive properties especially bonding strength.

Diversity of Benthic Organisms on Artificial Reef Structure

One of the methods of a marine rehabilitation program to accelerate the recovery of damaged coral reefs is to make artificial reefs as a new coral growth substrate. Interestingly, many benthic invertebrates overgrowth the artificial reef structures. The purpose of this study was to investigate the benthic organisms encrusting the artificial reefs including the cement and iron substrates. In June 2018, 10 artificial reef structures were deployed in 7-8m depth around Putri Island, Belinyu, Bangka Regency. The artificial reef structures were made in the form of an iron frame with a cement concrete weight. Colonization of sessile benthic organisms is generally marine invertebrates; Scleractinia corals, sponges, bivalves, hydrozoa, bryozoa, soft corals, gastropoda, crinoid, ascidian, and gorgonian. Natural recruited coral Pocillopora sp. was found in all artificial reef structures with colony sizes 3-8cm and surprisingly only coral pocillopora found adhered in iron frames and sinkers. The other benthic organisms are sponge, crinoid, and bryozoa with the number of densities are 2 organism/m2. Meanwhile, the lowest benthic density are groups of Mollusc and Ascidian with organism/m2. The type of succession that occurs in this research was likely a primary succession. The preference of benthic organisms among reefs appeared to be related to the proximity of natural hard-bottom habitat and type of iron and coating materials.

Transformation of Cyanobacterial Biomolecules by Iron Oxides During Flash Pyrolysis: Implications for Mars Life-Detection Missions.

Answering the question of whether life ever existed on Mars is a key goal of both NASA's and ESA's imminent Mars rover missions. The obfuscatory effects of oxidizing salts, such as perchlorates and sulfates, on organic matter during thermal decomposition analysis techniques are well established. Less well studied are the transformative effects of iron oxides and (oxy)hydroxides, which are present in great abundances in the martian regolith. We examined the products of flash pyrolysis-gas chromatography-mass spectrometry (a technique analogous to the thermal techniques employed by past, current, and future landed Mars missions) which form when the cyanobacteria Arthrospira platensis are heated in the presence of a variety of Mars-relevant iron-bearing minerals. We found that iron oxides/(oxy)hydroxides have transformative effects on the pyrolytic products of cyanobacterial biomolecules. Both the abundance and variety of molecular species detected were decreased as iron substrates transformed biomolecules, by both oxidative and reductive processes, into lower fidelity alkanes, aromatic and aryl-bonded hydrocarbons. Despite the loss of fidelity, a suite that contains mid-length alkanes and polyaromatic hydrocarbons and/or aryl-bonded molecules in iron-rich samples subjected to pyrolysis may allude to the transformation of cyanobacterially derived mid-long chain length fatty acids (particularly unsaturated fatty acids) originally present in the sample. Hematite was found to be the iron oxide with the lowest transformation potential, and because this iron oxide has a high affinity for codeposition of organic matter and preservation over geological timescales, sampling at Mars should target sediments/strata that have undergone a diagenetic history encouraging the dehydration, dihydroxylation, and oxidation of more reactive iron-bearing phases to hematite by looking for (mineralogical) evidence of the activity of oxidizing, acidic/neutral, and either hot or long-lived fluids.

Simultaneous laser in-situ generation of graphene and micro-textures on ductile iron and their effects on tribological properties

In this study, a picosecond (ps) laser was employed to produce micro-textures on ductile iron in linearly processing leading to significantly higher processing efficiency. The fabrication efficiency was increased about 1331 times as compared to the more common spot-by-spot texturing method. The ps laser induced graphene (LIG) structures were simultaneously fabricated in dimples, which further enhanced the surface tribological properties. The surface micro-textures were produced by forming micro-dimples on the substrate via the laser ablation process. The dimple diameter and area density were regulated by varying the key laser processing parameters, and were experimentally correlated to the tribological properties of the ductile iron substrate. To understand deeper into the wear reduction mechanism of the micro-dimples, simulation models based on the Reynold equation were established, which analyze the flow field pressure distribution and demonstrate the hydrodynamic lubrication effect of the micro-textures. The ball-on disk friction test showed that the area density of the micro-textures had a significant influence on the friction and wear properties. The micro-texture with the area density of 20% and the average dimple diameter of 36 μm showed the highest volume wear rate reduction of 70% and the friction coefficient reduction of 20% respectively. The results indicated that the simultaneous laser in-situ generation of graphene micro-textures is an effective way of enhancing tribological properties on the ductile iron surface. It enhances the hardness of textured surfaces, thermal diffusion, debris capture ability, and fluid dynamic lubrication effect.

Tannic acid/clay hydrogel with time-dependent mechanical and adhesive performance enabled by molecular interaction evolution

The investigation on dynamic molecular mechanisms is of importance for developing smart materials for various applications. Herein, we report a dynamic tannic acid (TA)/clay hydrogel with time-dependent mechanical and adhesive performance, in which clay plays an important role in regulating the molecular interaction evolution of hydrogels. Specifically, TA mediated the assembly of clay to form networked hydrogels driven by TA-clay dynamic interactions. Afterwards, clay served as a pro-oxidant component accelerated covalent crosslinking reactions of TA. Consequentially, the hydrogel transitioned from reversible dynamic networks to irreversible covalent networks over the time. Owing to the molecular interaction evolution, storage modulus of hydrogels gradually increased from 22 Pa to 3235 Pa in 4 days. Adhesive strength between hydrogels and iron substrates increased from 3 MPa to 23 MPa in 12 h and then decreased to 5 MPa. This finding will be a general principle for designing dynamic materials with time-dependent physicochemical properties and will enable various applications such as customized temporary adhesives.

Oxygen Vacancy Defects and a Field Effect-Mediated ZnO/WO2.92 Heterojunction for Enhanced Corrosion Resistance.

The heterojunction constructed by tungsten oxide and zinc oxide materials can improve the problem of easy deactivation of electrons, which is a new and effective strategy for realizing anticorrosion. Here, the ZnO/WO2.92 heterojunction modified by oxygen vacancies (OVs) serving as the photoelectric conversion center was not consumed to provide continuous light-induced protection for steel, and the impedance value was increased by 185.35% compared to that of epoxy resin after 72 h of corrosion. The enhanced anticorrosion activity was due to OV modification leading to oxygen adsorption and electron capture, which inhibited the cathodic corrosion reaction and effectively hindered electron transport. Additionally, the localized surface plasmon resonance effect produced by OVs improved light utilization efficiency and increased electron density, which enabled numerous photoelectrons to gather on the surface of the iron substrate to reduce the corrosion rate of metals. Besides, the cascade effect of the ZnO/WO2.92 heterojunction promoted the transfer of e-/h+ to form an electric field that allowed the directional flow of electrons to inhibit the anode dissolution process. Thus, exploring the corrosion reaction involving OVs and heterojunction structures was of great significance to the development of nonsacrificial and efficient anticorrosion materials.

Low-Mass Liquid Crystalline Materials Blended in Recycled Thermoplastic Polyester Elastomer for Corrosion Inhibitor Application

In this work, the development and application of multicomponents obtained from recycled polyethylene terephthalate (r-PET) waste and monotropic liquid crystals as anticorrosion coatings are reported. The r-PET raw material was alcoholyzed and reproduced as a thermoplastic polyester elastomer (TPEE) with different amounts (n%, n = 0, 1, 3, and 5) of 1,6-hexanediamine (HDA). Then, a fluorine-containing liquid crystal (4-cyano-3-fluorophenyl 4-ethylbenzoate (4CFE)) was incorporated into the TPEE mixture via solvent blending to modify and enhance the water resistance. The adhesion behavior of the coating on glass and iron substrates was evaluated by cross-cut tests and immersion tests in aqueous NaCl. In the corrosion resistance measurements, all of the coating samples fabricated with 10 ± 1 mm thickness were less active toward electrochemical corrosion (PEF% > 99%) than the bare iron plate, indicating that our work provided better protection against corrosion of the iron plate.

Structure and mechanical properties of WC interlayers between iron substrate and thin tungsten plate

To improve the mechanical properties of cast iron (Fe) surface, tungsten carbide (WC) interlayers were fabricated by the in-situ reaction at 1050 °C and 1100 °C for different heat-treatment times. The structure and phase composition of the layer were investigated via scanning electron microscopy, transmission electron microscopy and X-ray diffraction analyses. The effect of vacuum atmosphere temperature on the interfacial structure of sample was discussed. The results indicate that the thickness of the layer, consisting of a dominant WC phase, has a range of 11.12 ± 1.32 to 51.51 ± 1.36 μm obtained at 1050 °C. Based on growth kinetics, the diffusion coefficient of carbon in the WC layer is calculated to be 1.57×10−9 cm2/s. Furthermore, under the load of 100 mN, nanoindentation tests demonstrate that values of hardness and elastic modulus of the layer are approximately 25.09–39.74 GPa and 649.54–834.17 GPa, which are 2.4–3.8 times and 3.5–4.4 times larger than that of the substrate, respectively. By means of Vickers indentation mode using the available equation for Palmqvist crack system, the value of the calculated fracture toughness for the layer changes in a range from 2.55 ± 0.14 to 6.34 ± 0.54 MPa⋅m1/2, and the value decreases with the increasing load. This result may be caused by the reason that the fracture resistance also strongly depends on the applied load.

Simulation of the incubation time for the formation of (FeB/Fe2B) bilayer on pure iron

Abstract In this work, a mathematical model was used in order to study the growth kinetics of (Fe2B/FeB) bilayer during bori-ding process basing on the second Fick’s law and mass balance equation. The run of the numerical simulation allowed calculating the incubation time (τ) of each boronized layer (Fe2B or FeB), and estimating the effect of this parameter on the growth of the boronized layer. The boride incubation time for forming the FeB or Fe2B layer on the pure iron substrate was incorporated into the present mathematical model. To simulate the value of the growth rate constant and the incubation time for the bilayer configuration, the experimental data available in the literature concerning the boronizing of pure iron were considered. Based on the experimental and simulation results, it was shown that the incubation time decreases with increasing temperature in the FeB and Fe2B phases. It was concluded from this study that the thickness of each boride layer depended on its growth rate constant and on another parameter kτ which was the rate constant of incubation time.The obtained results confirmed the validity of the present mathematical model and gave a good estimate of the incubation time during the formation of each boride layer as well as formulated the variation of this parameter with a mathematical equation. Furthermore, the comparison of experimental data with the simulated results of boronized layer thickness allowed to validate the present model.

Erosion behaviour of HVOF sprayed Alloy718-nano Al2O3 composite coatings on grey cast iron at elevated temperature conditions

The current study investigates the erosion performance of high-velocity oxy-fuel (HVOF) sprayed Alloy718-nanoAl2O3 composite coatings on cast iron. The effect of the addition of nano-Al2O3 reinforcement in the Alloy718(AL718) matrix was studied by varying the nano-Al2O3 content from 10wt% to 30wt%. Initially, the microstructural and phase analysis of the developed coatings was conducted. The as-deposited coating in the AL718matrix showed the well-scattered nano-Al2O3particles in the AL718 matrix. The highest micro-hardness was found to be1296 ± 40 HV0.2 for Alloy718%-30%nanoAl2O3 (30N) coating, which is around 3.56 times higher than the grey cast iron (GCI) substrate. The air-jet erosion test was conducted at 900 °C at both low and high degrees impact angles (30° and 90°). The 30N coating showed the maximum erosion resistance due to the higher micro-hardness and fracture toughness amongst all the deposited coatings. The increased nano-Al2O3 content enhanced the erosion resistance of the coatings by providing dispersion strengthening to the matrix. The formation of cavities, ploughing, lip formation and micro-cutting were observed in all deposited coatings at 30° impact angle as wear mechanism. The formation of cracks, craters and pullout was the responsible wear mechanism at 90° impact angle for all deposited coatings. The developed coatings showed mixed behaviour of ductile and brittle mode of failure at 30° impact angle and brittle mode of failure at 90° impact angle for 30N coating.

Unveiling microbiologically influenced corrosion engineering to transfigure damages into benefits: A textile sensor for H2O2 detection in clinical cancer tissues

With the ongoing increasing research interests in economical electrocatalysts, transforming damages into benefits to prepare useful electrode materials is an ideal strategy to achieve the goals. Corrosion engineering converts harmful corrosion processes into high performance catalyst nanostructures. In this work, we develop an inexpensive, scaled-up corrosion engineering strategy for value-added transformation of low-cost iron substrates into highly efficient Cu-Fe(OH)2-FeS deposited on polyester cloth fabric (PCF) flexible electrode by a process of electroless plating combined with microbes assisted corrosion product. The anaerobic sulfate reducing bacteria (SRB) converting sulfate to sulfide play vital role to carry out the construction of Cu-Fe(OH)2-FeS/PCF electrode which reveals high electrochemical sensing performance for H2O2 with broad linear range and low detection limit of 0.2 nM (S/N = 3). The enhanced activity arises from densely deposited nanosheets of transition metals oxides/hydroxides, plethora of surface active sites and synergistic effect between Cu-Fe(OH)2 and FeS species. More importantly, the S2− ions serving as co-catalyst have been found to continuously fuel electrons during the reduction of Fe(III) and Cu(II) which accelerate the redox cycles of Fe(III)/Fe(II) and Cu(II)/Cu(I) further enhancing electrocatalytic H2O2 reduction. With high sensitivity achieved, the Cu-Fe(OH)2-FeS/PCF electrode has also been practically applied in real-time in vitro tracking of H2O2 excreted from different normal and human brain cancer cell lines as well as in situ sensitive detection of H2O2 released from human brain tumor tissues. This work presents a good way to bridge up the gap between annoying traditional corrosion engineering and emerging electrochemical technologies.

N‐Doped FeS2 Achieved by Thermal Annealing of Anodized Fe in Ammonia and Sulfur Atmosphere: Applications for Supercapacitors

Iron pyrite (FeS2) has attracted intense research interest for low-cost supercapacitor applications due to its improved electrical conductivity in comparison with its oxide counterpart. Herein, we demonstrate a novel strategy for the facile synthesis of N-doped FeS2 nanosphere films on iron substrate, which includes anodization of Fe and subsequent thermal annealing treatment. The rod-like ferrous oxalate films are obtained by anodization of Fe in an oxalic acid electrolyte. Then, nanosphere-like N-doped FeS2 films can be readily achieved through thermal annealing of ferrous oxalate films under ammonia (NH3) atmosphere in the presence of sulfur powder. The N-doped FeS2 electrode fabricated without the need for any binder shows superior rate capability and enhanced specific capacitance compared to the pristine FeS2. The prepared N-doped FeS2 nanosphere electrode displays a high specific capacitance of 238.2 mF cm−2 at 3 mA cm−2 over a potential range of −1.2 to 0 V and good cycling stability with a capacitance retention of 84.3% after 5000 cycles.

Effect of substrate on operation performance of ecological floating bed for treating simulated tailwater from wastewater treatment plant

ABSTRACT Ecological floating bed (EFB) with assembling substrate can effectively enhance the removal of nitrogen. However, the influence of the substrate on the operation process of the EFB remains unclear. In this study, EFBs with zeolite, sponge iron and complementary substrates (zeolite and sponge iron) were established to explore the effects of different substrates on nitrogen removal efficiency, physical and chemical properties and plant growth. Complementary substrates could efficiently and stably remove nitrogen from tailwater. The concentration of ammonium nitrogen (NH4 +-N), nitrate nitrogen (NO3 --N) and total nitrogen (TN) in the effluent during the operation were lower than 1, 1 and 1.5 mg/L, respectively. Additionally, complementary substrates had no inhibitory effect on plant growth, and those substrates had little influence on the distribution of dissolved oxygen (DO), the oxidation–reduction potential (ORP) and the pH value. The zeolite increased the NH4 +-N removal and promoted the growth of plant stems and roots. However, sponge iron could effectively remove the NO3 --N in the water body and inhibit the growth of plants. These results can provide a basis for the EFB in the reuse of the tailwater of wastewater treatment plant.

RESEARCH ON TRIBOLOGICAL BEHAVIOR OF Cr3C2-25NiCr-COATED BRAKE DISC

This research presents the results of a comparative study to determine the friction and wear behavior of the high-velocity oxygen fuel (HVOF)-sprayed Cr3C2-25NiCr-coated brake disc. The friction and wear behavior of Cr3C2-25NiCr-coated brake disc (CD) and the sliding against the commercial brake pad were examined on a laboratory-scale disc-pad dynamometer and the findings were compared with those of reference brake disc (UCD). The coating was deposited by means of the HVOF process on a grey cast iron (GCI) substrate. Braking tests were performed according to the SAE-J2430 test standard. The as-sprayed coating was characterized for their microstructure and phase constitution by SEM/EDS and XRD. The results reveal that the coefficient of friction (COF) of CD and UCD varies between 0.42–0.57 and 0.44–0.62, respectively. The Cr3C2-25NiCr-coated brake disc was found to have exhibited a comparable COF and lower wear loss to the reference brake disc. The addition of a ceramic phase to the brake disc coating played an important role in reducing the COF to an acceptable range and improving the sliding wear resistance of the brake pair.

Microstructural characterization and electrochemical corrosion behaviour of HVOF sprayed Alloy718-nanoAl2O3 composite coatings

In the present research work, microstructural evolution and corrosion behaviour of the high-velocity oxy-fuel (HVOF) sprayed Alloy718-nano Al2O3 composite coatings were examined. These coatings were deposited on the grey cast iron (C.I) substrate. The agglomerated nano-Al2O3 reinforcement particles in three different proportions (10, 20 & 30 wt.%) were varied in the Alloy718 based matrix to develop the composite coatings. Corrosion testing of bare and coated samples was carried out at an ambient temperature in the NaCl solution (3.5 wt.%) using the potentiodynamic polarization technique. The polarization resistance (Rp), the corrosion efficiency and the corrosion rate (CR) were measured for the developed coatings and substrate. The surface morphology of as-sprayed and corroded coatings was studied using scanning electron microscopy coupled with energy-dispersive spectroscopy. X-ray diffraction analysis was conducted to determine the different phases present in the feedstock powders and as-sprayed coatings. The as-sprayed coatings were seen with nano-Al2O3 particles uniformly embedded in the melted Alloy718 matrix. The coating with 30 wt.% nano-Al2O3 particles revealed the maximum microhardness of about 1296 ± 21 HV0.2 compared to other coatings. The results showed that the corrosion resistance of the composite coatings increases with the increase in alumina particles in the Alloy718 matrix. The 30 wt.% alumina coating showed maximum resistance to corrosion due to the maximum decrease in the electrochemically active area associated with this coating.

Mechanistic study on H2S and subsequent O2 adsorption on iron oxides and hydroxides

Iron oxides are useful adsorbents for the alternating removal of H2S and O2 from natural gas and biogas. In this context, the present paper aims to elucidate the mechanisms behind the consecutive adsorption of both gases. Three different types of iron oxides/hydroxides were investigated using in-situ infrared and Raman spectroscopy as well as ex-situ X-ray diffraction.The spectroscopic studies confirm that H2S is adsorbed on all kinds of surface sites, particularly at Brønsted and Lewis acid sites. However, on the Brønsted acid sites, H2S undergoes dissociation, leading to the formation of SH−, S2− and H2O. This mechanism substantiates the high H2S uptake capacity of the OH-containing adsorbents. Prolonged H2S adsorption provokes an increasing consumption of the adsorbent, yielding amorphous FeS and elemental sulfur, with hydrogen sulfate and sulfate species also forming as by-products.Furthermore, the subsequent O2 adsorption on the sulfidized samples causes some re-formation of the initial adsorbent with the simultaneous production of elemental sulfur and sulfate entities. As a result of the mechanistic understanding gained in this study, it is concluded that the complete regeneration of the iron substrates is almost impossible due to their substantial degradation, associated with partial conversion into sulfates and the deposition of sulfur.

Biocompatible Catechol‐Functionalized Cellulose‐Based Adhesives with Strong Water Resistance

AbstractNumerous traditional adhesives have good adhesion in dry environments. However, non‐environmental‐friendliness and poor water resistance largely limit their practical applications. To prepare biocompatible adhesives with strong water resistance and adhesion strength, in this paper, catechol‐functionalized cellulose‐based adhesive polymers are synthesized by grafting N‐(3,4‐dihydroxyphenethyl)methacrylamide and methyl methacrylate onto cellulose chain through atom transfer radical polymerization (ATRP). The successful synthesis of the catechol‐functionalized cellulose‐based adhesive polymers is confirmed by FTIR and 1H NMR. The different characteristics of the adhesive polymers, such as thermal stability, swelling ratio, biocompatibility, and adhesion strength are investigated. Strong water resistance on various substrates is realized in underwater environment for the catechol‐functionalized cellulose‐based adhesive with addition of Fe3+. The adhesion strength and thermal stability are enhanced when the catechol content is increased. The adhesive with catechol content of 25.4% shows the adhesion strength of 0.45 MPa for iron substrate in underwater environment. In addition, the adhesive with addition of Fe3+ exhibits excellent adhesion in dry environment, with maximum adhesion strength of 3.50 MPa for iron substrate. The cell culture test shows that the adhesive polymers have excellent biocompatibility. The biocompatible adhesives with strong water resistance have potential application in electronic, wood, and building fields.

Delivery and effects of proton pump inhibitor on anaerobic digestion of food and kitchen waste under ammonia stress

Ammonia stress changes microbial metabolism of anaerobic digestion and decreases methane yield, where proton pump overactivated by free ammonia suggested to be the centre of the metabolism changes in anaerobic digestion under ammonia stress. The work demonstrated that proton pump inhibitor (PPI) could alleviate the overactivated proton pump and mitigate ammonia inhibition. Its impacts on iron transporter, substrates uptake, and energy conservation were investigated in anaerobic digestion treating food and kitchen waste. The PPI formed a stimuli-responsive drug delivery system driven by pH for the more inhibited microbe (p < 0.01), confirmed by FE-SEM/EDS and high throughput sequencing, implying the PPI was activated at inhibited microbe more than mixed liquor. Consistent microbial population increase observed in syntrophs and methanogens, who utilized the substrates for high yielding pathway and facilitated the energy sharing by direct interspecies electron transfer. These results demonstrated PPI could recovery methane production and could mitigate fatty-acid accumulation under high ammonia stress by delivery and activation in acetoclastic methanogen.

Tribological Behavior of Ni-Based WC-Co Coatings Deposited via Spray and Fuse Technique Varying the Oxygen Flow

The tribological behavior of Ni-based WC-Co coatings is analyzed. The coatings were deposited on gray cast iron substrates in a spray and fuse process using SuperJet Eutalloy deposition equipment, varying the oxygen flow conditions in the flame. The crystallographic structure of the coatings was characterized via the X-ray diffraction (XRD) technique. The microhardness was measured on the surface and in cross sections of the coatings by means of a Knoop microhardness tester. The topography and the morphological characteristics of the coatings and the tribo-surfaces were examined using scanning electron microscopy (SEM) and confocal microscopy, while the chemical composition was measured by means of energy-dispersive X-ray spectroscopy (EDS). The tribological behavior of the coatings was examined via a cohesion-adhesion scratch test, using cross sections of the coatings. Furthermore, two wear tests were carried out, using the pin-on-disk method under ASTM G99 standard and an ASTM standard G65 sand/rubber wheel abrasion wear test. The wear of the coatings showed a close relationship to the porosity in the metal matrix; since then, in the abrasive wear test, a high porosity is related with lower hardness in the coatings; likewise, a low hardness is related with a high wear.

N-Nitrosation Mechanism Catalyzed by Non-heme Iron-Containing Enzyme SznF Involving Intramolecular Oxidative Rearrangement.

The non-heme iron-dependent enzyme SznF catalyzes a critical N-nitrosation step during the N-nitrosourea pharmacophore biosynthesis in streptozotocin. The intramolecular oxidative rearrangement process is known to proceed at the FeII-containing active site in the cupin domain of SznF, but its mechanism has not been elucidated to date. In this study, based on the density functional theory calculations, a unique mechanism was proposed for the N-nitrosation reaction catalyzed by SznF in which a four-electron oxidation process is accomplished through a series of complicated electron transferring between the iron center and substrate to bypass the high-valent FeIV═O species. In the catalytic reaction pathway, the O2 binds to the iron center and attacks on the substrate to form the peroxo bridge intermediate by obtaining two electrons from the substrate exclusively. Then, instead of cleaving the peroxo bridge, the Cε-Nω bond of the substrate is homolytically cleaved first to form a carbocation intermediate, which polarizes the peroxo bridge and promotes its heterolysis. After O-O bond cleavage, the following reaction steps proceed effortlessly so that the N-nitrosation is accomplished without NO exchange among reaction species.